Endless

[FanofSMBX]

I'm afraid a REALLY long autoscroll room is the best you can get like this.
(FYI I can make other sections autoscroll with the magic of LunaDLL)

[SuperWingedYoshi]

There's nothing such as an endless section, sadly. All you can do is to make a really, really long autoscroll section, make a layer which has the end of the level (like a warp or an NPC), attach the end of the level to the boss, make the end of the level invisible at the start of the level, and make the end of the level appear when the event "Death" occurs, as well as stopping the autoscroll with the same event. That's the closest you can get without LunaDLL.